This invention pertains to a method for semi-continuous emulsion polymerization, which produces a bimodal product in volumes greater than that of a single reactor, and which requires a minimum investment of additional equipment.
A bimodal polymer is a polymer wherein at least 90 percent of the polymer is present in two populations of particle mode, wherein each mode has a different particle size. The second mode creates a packing phenomena wherein the smaller mode fills the gaps between the larger mode, thus resulting in a polymer containing significantly higher solids than a unimodal polymer. Several patents describe the manufacture of bimodal polymers by various routes. However, because of the need to grow the second particle mode in the presence of an initial, or first mode, it has been generally accepted that the second particle mode must be grown in its entirety in the presence of the first mode, using a multi-step process. U.S. Pat. No. 4,254,004 discloses a two-step latex polymerization of ethylenically saturated monomer wherein the monomer feed rate exceeds the monomer polymerization rate in the first step until the Trommsdorff exotherm occurs, followed in the second step wherein the monomer feed rate is slower than the monomer polymerization rate to produce in-situ bimodal latex polymer particles. This process produces only a single reactor volume per batch.
In recent years, it has been discovered that unimodal emulsion polymerization can be carried out using a semi-continuous process. U.S. Pat. No. 4,946,891 discloses an aqueous copolymerization carried out on a semi-continuous basis using a single continuously stirred tank reactor in which, as is conventional, the materials being copolymerized (monomers) and the materials used in the copolymerization are supplied slowly to a reactor while maintaining conditions causing copolymerization to proceed until the total supply is in the range of from about 1.5 to about 5 reactor volumes. As materials are added to the reactor, a corresponding amount of the reactor contents spills out of the reactor into one or more vessels in which the copolymerization reaction is completed. This process produces only a unimodal polymerization product.
The problem addressed by the invention is the provision of a process for semi-continuous emulsion polymerization, which produces a bimodal product in volumes greater than that of a single reactor, and which requires a minimum investment of additional equipment.
In a first aspect of the invention there is provided a method of producing a bimodal polymer product, by semi-continuous, aqueous polymerization, composed of supplying to a single continuously stirred tank reactor, materials used in the polymerization, including water, and at least one ethylenically unsaturated monomer; creating in the reactor, conditions causing polymerization of a first polymer mode to proceed; adding a surfactant or an emulsion polymer having a diameter different from that of the particles in the first mode at that point in the reaction, to initiate growth of a second polymer mode; continuing the supply, under the conditions, producing a bimodal polymer product; continuing the supply to the reactor, while simultaneously removing part of the bimodal polymer product from the reactor; collecting the removed bimodal polymer product in at least one separate vessel, continuing the supply, and maintaining the conditions until the total supply is at least 1.05 reactor volumes and less than to 2 reactor volumes; completing the polymerization reaction in the separate vessel; and combining together all of the bimodal polymer product.
In a second aspect of the invention there is provided a bimodal composition, prepared by semi-continuous, aqueous polymerization composed of supplying to a single continuously stirred tank reactor, materials used in the polymerization, including water, and at least one ethylenically unsaturated monomer; creating in the reactor, conditions causing polymerization of a first polymer mode to proceed; adding a surfactant or an emulsion polymer having a diameter different from that of the particles in the first mode at that point in the reaction, to initiate growth of a second polymer mode; continuing the supply, under the conditions, producing a bimodal polymer product; continuing the supply to the reactor, while simultaneously removing part of the bimodal polymer product from the reactor; collecting the removed bimodal polymer product in at least one separate vessel, continuing the supply, and maintaining the conditions until the total supply is at least 1.05 reactor volumes and less than to 2 reactor volumes; completing the polymerization reaction in the separate vessel; and combining together all of the bimodal polymer product.
The process in this invention produces bimodal polymer particles. The bimodal product is made up of a mixture of polymer particles of two size distributions, wherein the diameter of the larger particles may be 3.5 to 30 times the diameter of the smaller distribution particles, and preferably 5 to 15 times larger than the smaller particles, as measured by weight average particle size.
The semi-continuous aqueous polymerization process is carried out by first supplying to a single continuously stirred tank reactor the materials used in the polymerization. These materials include water, and at least one ethylenically unsaturated monomer, and may include an emulsifier, catalyst and/or a polymer seed. By semi-continuous, herein is meant some of the reactants are charged to the reactor at the beginning of processing, and the remaining reactants are fed continuously as the reaction progresses while some of the product is simultaneously withdrawn from the reactor. By continuously stirred, herein is meant the reactants are agitated during processing, providing mixing and creating a substantially uniform composition within the reactor. By tank reactor, herein is meant a vessel with inlet and outlet pipes, equipped with some means of agitation and provisions for heat transfer (for example jacket, external or internal heat exchangers), and which can accommodate either batch or continuous operations over wide ranges of temperatures or pressures. By polymer seed, herein is meant, a polymer composition whose particle size predefines the diameter of the first polymer mode of the bimodal product. This invention contemplates that the addition of water and monomer to the reactor may include the addition of the water followed by neat monomer, or the addition of water followed by a monomer emulsion, or the addition of a mixture of water and a small amount of monomer followed by the addition of a monomer emulsion.
At least one ethylenically unsaturated monomer may be selected from amides such as (meth)acrylamide, propenamide, dimethylacrylamide; esters such as methyl acrylate, ethyl acrylate, butyl acrylate, propyl acrylate, 2-ethylhexyl acrylate, hydroxyethyl acrylate, methyl methacrylate, hydroxyethyl methacrylate, hydroxymethyl acrylate, hydroxymethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, allyl methacrylate, diallyl phthalate, 1,3-butylene glycol dimethacrylate, 1,6-hexanedioldiacrylate, vinyl acetate, vinyl proprionate, or other vinyl esters; nitriles such as acrylonitrile; and the like, and combinations of the foregoing. Other suitable ethylenically unsaturated monomers may include vinyl monomers such as vinyl chloride, vinylidine chloride, vinyl acetate and N-vinyl pyrollidone, divinyl benzene; styrene or alkyl-substituted styrene, butadiene; and the like, and combinations of the foregoing. Examples of ethylenically unsaturated carboxylic acid monomers that are useful in this invention include acrylic acid, methacrylic acid, fumaric acid, crotonic acid, maleic acid, itaconic acid, and combinations of two or more such acids. Preferably, the ethylenically unsaturated carboxylic acid is acrylic acid. Preferably, a stabilizing monomer, such as an acid containing monomer is used to stabilize the emulsion polymer. Examples of specific stabilizing monomers include the monomers listed above as examples of ethylenically unsaturated carboxylic acid monomers.
Catalysts which may be used to cause free radical polymerization of the above monomers include, a thermal initiator, or a redox initiator system composed of an oxidizing agent and a reducing agent. Examples of suitable oxidizing agents include ammonium persulfate, alkali metal persulfates; perborates; peracetates; percarbonates; peroxides, for example hydrogen peroxide, cumene hydroperoxide, dibenzoyl peroxide, diacetyl peroxide, dodecanoyl peroxide, di-t-butyl peroxide, dilauroyl peroxide, bis(p-methoxy benzoyl) peroxide, t-butyl peroxy pivilate, and dicumyl peroxide; isopropyl percarbonate; di-sec-butyl peroxidicarbonate, and the like, and mixtures thereof. Examples of suitable reducing agents include alkali metal and ammonium salts of sulfur-containing acids such as sodium sulfite, bisulfite, metabisufite, thiosulfite, sulfide, hydrosulfide, or dithionite; sulfinic acids, such as alkylsulfinic acids, aryl sulfinic acids, and hydroxyalkyl sulfinic acids, and 2-hydroxy-2-sulfinatoacetic acid; amines such as ethanolamine; glycolic acid; glyoxylic acid hydrate; ascorbic acid; isoascorbic acid; lactic acid; glyceric acid; malic acid; tartaric acid and salts of the preceding acids, salts of the preceding acids, and the like, and mixtures thereof. Thermal initiators can be used which decompose or become active at the polymerization temperature. Examples of suitable thermal initiators include those compounds listed above as oxidizing agents.
Emulsifiers which are typically used may be any anionic, nonionic, or cationic surfactant, soap, or the like, which are well known in the art, and stable at the pH of the bimodal latex. Examples of specific emulsifiers include alkyl sulfates, alkyl sulfosuccinates, alkyl aryl sulfonates, xcex1-olefin sulfonates, quaternary ammonium salts, amine salts, fatty or resin acid salts, nonyl or octyl phenol reaction products of ethylene oxide and the like. Examples of specific surfactants include sodium lauryl sulfate, sodium sulfosuccinates such as sodium dimethylamyl sulfosuccinate, sodium dodecyl diphenyloxide disulfonate and the like. The amount of emulsifier present is sufficient to obtain an aqueous emulsion of the monomers.
Optional chain transfer agents include mercaptans such as the alkyl and/or aralkyl mercaptans. Examples of specific chain transfer agents include n-octyl mercaptan, n-dodecyl mercaptan, t-octyl mercaptan, t-dodecyl mercaptan, tridecyl mercaptan, tetradecyl mercaptan, hexadecyl mercaptan and the like, as well as mixtures thereof.
Conditions are created in the reactor which cause polymerization of a first polymer mode to proceed. The reactor is usually initially heated to establish the desired reaction temperature for production of a first polymer mode. The reaction temperature may range from 30xc2x0 C. to 150xc2x0 C., preferably from 60xc2x0 C. to 95xc2x0 C. Cooling may be utilized. After 5 to 95 percent, preferably 15 to 60 percent, of the reactor feeds have been charged to the reactor, a surfactant, or an emulsion polymer is added to the reactor to initiate the growth of a second polymer mode. The surfactant may be selected from the compounds listed above as examples of emulsifiers. If an emulsion polymer is used to initiate the growth of the second mode, it must have a diameter which differs from that of the particles in the first mode at that point in the reaction. The supply of materials used in the bimodal polymerization is continued under the above-mentioned conditions. Additional catalyst may be added to the reactor. The rate of the reaction is determined by the type of catalysts that are selected, and the reaction temperature.
In one embodiment of the invention, the supply of materials used in the bimodal polymerization is continued, while the bimodal product is simultaneously, continuously or not, removed from the reactor. Removal of the bimodal product may begin as soon as the second mode initiating surfactant or emulsion polymer has been added, preferably when the reaction mixture takes up 90 to 99 percent of the reactor capacity, and most preferably when the reactor is full. By full, herein is meant the maximum operating volume recommended by the reactor manufacturer. The bimodal product is removed from the reactor, at a rate at least as fast as the rate of the reactor feeds.
In another embodiment of the invention, the supply of materials used in the bimodal polymerization is continued until the reactor is full, at which time, the reactor feeds are stopped. A portion of the reactor mixture is removed from the reactor and transferred to a separate vessel. After the transfer is complete, the feeds to the reactor are resumed.
The supply to the reactor of materials used in the bimodal polymerization is stopped when the total supply is at least 1.05 reactor volumes and less than 2 reactor volumes, preferably at least 1.2 and less than 1.9 reactor volumes, and most preferably at least 1.4 and less than 1.8 reactor volumes. Processing of more than 2 reactor volumes using this invention may result in an increased number of polymer modes. The total bimodal product is transferred to at least one separate vessel.
The polymerization reaction is completed in the separate vessel(s). By completed herein is meant that the polymerization is continued until the residual monomer has reached a desired level of conversion, such as at least 95%, preferably at least 98%, most preferably at least 99.5% of the monomer has been converted to polymer, and the ratio of solids to water is 70:30 to 20:80. By solids, herein is meant, a composition including polymer, and solid fragments from surfactant, catalyst, activator or any other nonvolatile materials used in the polymerization reaction. By separate vessel, herein is meant a container that provides the capability to feed additional materials to the bimodal product. All of the bimodal product is combined together. Provision should be made to ensure the uniform distribution of the bimodal particles, whether by mixing, during transportation, or by other means. It is also contemplated that functional additives may be added to the bimodal product. By functional additives, herein is meant biocides, defoamers, thickeners, and the like.
In one embodiment of this invention, the emulsion polymerization product may be a multimodal polymer product with three or more particle size modes.
The bimodal emulsion polymer of this invention is especially useful in aqueous latex paints. In addition to paints, the bimodal polymer particles of this invention are useful for providing improved physical properties in paper coatings, leather coatings, plastics, adhesives, roofing materials, nonwoven and paper saturants, and the like.
The following examples are presented to illustrate the invention and the results obtained.